1. Field of the Invention
The present invention relates to a laminated balance filter having a multilayer body including a plurality of dielectric layers and a plurality of electrode layers.
2. Description of the Related Art
In recent years, high-frequency band pass filters having a reduced size and cost include a plurality of LC resonators in a multilayer body in which a dielectric layer and an electrode layer are laminated. An example of such a multilayer band pass filter is disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447. The configuration of a multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447 will be described with reference to FIG. 1.
FIG. 1 is a circuit diagram of the multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447. A multilayer band pass filter 1 includes an unbalanced input 2, balanced outputs 3A and 3B, and a band pass filter unit 4 arranged between the unbalanced input 2 and the balanced outputs 3A and 3B. The band pass filter unit 4 includes three resonators 40 that are ½ wavelength resonators 40, both ends of which are open, and arranged next to one another. An input resonator 401 is arranged at a location closest to the unbalanced input 2. The unbalanced input 2 is directly connected to the input resonator 401. A balanced output ½ wavelength resonator 41A is arranged at a location closest to the balanced outputs 3A and 3B. The balanced outputs 3A and 3B are directly connected to the balanced output ½ wavelength resonator 41A. The input resonator 401 and an intermediate resonator 40M are electromagnetically coupled to each other, and the intermediate resonator 40M and the balanced output ½ wavelength resonator 41A are also electromagnetically coupled to each other. In addition, capacitors C are provided between the individual open ends of the three resonators 40 and a ground.
In the multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447, a plurality of λ/2 resonators, both ends of which are open, are disposed on the same substrate surface, and, by coupling the resonators to one another, a balanced-unbalanced conversion function and filter characteristics are obtained.
In a filter that has the balanced-unbalanced conversion function, the balanced terminal thereof is typically connected to an IC that inputs and outputs a signal in a balanced manner. In such a case, it is necessary to convert impedance between an unbalanced terminal and a balanced terminal. However, since balance characteristics are obtained using stripline coupling in the structure illustrated in Japanese Unexamined Patent Application Publication No. 2005-45447, it is difficult to selectively design the impedance.
Since a ½ wave length stripline is used to balance the characteristics, it is difficult to reduce the size thereof. In addition, if an impedance converting circuit is provided, an area occupied by the impedance converting circuit increases, and thus, it is difficult to reduce the size of the filter.